1. Field of the Invention
The present invention relates to optical disks, optical disk apparatuses, methods for recording optical disks, and methods for reproducing optical disks, and more particularly, to an optical disk which records information at a high density and its related apparatuses. The present invention improves the recording density of an optical disk in the radial direction by densely forming tracks such that inter-code interference is generated between adjacent tracks, and by forming pits or marks so as to cancel the inter-code interference.
2. Description of the Related Art
A conventional optical disk is formed such that crosstalk caused by interference between codes from adjacent tracks is effectively avoided.
In a compact disc, which is reproduced in a reproduction apparatus for such an optical disk, a track pitch .delta. is set to 1.6 .mu.m. In a compact disc player, the wavelength .lambda. and the numerical aperture NA of a reproduction optical system are set against the track pitch .delta. such that the size of an area image, namely 1.22.lambda./(2NA), satisfies expression (1). EQU .delta.&gt;1.22.lambda./(2NA) (1)
With this setting, the compact disc player reproduces data recorded in a compact disc, with crosstalk between adjacent tracks being effectively avoided.
In contrast, in a magneto-optical disk apparatus, a so-called land/groove recording method is applied. With effective use of the depth-of-focus in a reproduction optical system, crosstalk between adjacent tracks is effectively avoided and information is recorded at higher density. (This method is described, for example, in "LAND/GROOVE RECORDING IN MO SYSTEMS," A. Fukumoto et al., Technical Digest, Optical Data Storage 1995, PP. 117-118.)
In a magneto-optical disk, since grooves for guiding a laser beam and lands formed between grooves have different depths from the surface of the disk, laser beams are condensed separately on grooves and lands, and changes in light returned from marks formed on the grooves and the lands are effectively avoided. In recording, laser beams are condensed separately on grooves and lands, and increases in temperature to the magnetic Curie point or more at the grooves and the lands are effectively avoided. With this configuration, grooves and lands are densely formed at an interval of the size of an area image or less, and the desired data can be recorded into and reproduced from grooves and lands.
In such an optical disk, however, recording density in the track direction is smaller than that in the circumferential (line) direction. In a compact disc, for example, recording density in the line direction is about 0.6 .mu.m/bit whereas the track pitch is 1.6 .mu.m. This means that recording density in the line direction and that in the track direction differ by a factor of 2.7.
To solve this problem, a method of land/groove recording, which is applied to a magneto-optical disk, can be considered. It is, however, practically difficult to apply the method to an optical disk which records information with the use of pits, such as a compact disc. Since a pit and a groove are formed by the same method, it is difficult to form a pit among grooves. In addition, for effectively preventing crosstalk from being generated, grooves must be maintained in their ideal shape, and hence mass production is difficult.